1. Field of the Invention
The invention relates to a transmission output amplifier and, more particularly, to a transmission output amplifier which is suitable for an amplifier of a small radio telephone.
2. Description of the Related Art
Hitherto, in a transmission output section of a mobile telephone of a CMTS (Cellular Mobile Telephone System) or a portable radio telephone, when dividing an output from the maximum output of 3 W to the minimum output of 5 mW into seven stages and controlling in a state in which an excitation input signal is set to be constant, there is used an APC (Auto Power Control) of a transmission output such that a power source voltage of a power amplifying section is changed to thereby adjust an output level.
Hitherto, as such a kind of transmission output amplifier, there is known an amplifier shown in FIG. 3. In FIG. 3, the transmission output amplifier comprises an exciting section A and a power amplifying section B. The exciting section A receives a signal from the front stage by an excitation input terminal 51. The signal is amplified by transistors 52 and 55. The amplified signal is supplied to the power amplifying section B through a BPF 56 and a coupling terminal 60. The signal is power amplified by transistor 62 and 65 of the power amplifying section B and is output from an output terminal 68. When an output signal of the exciting section A is applied to the power amplifying section B, an excitation output of tens of mW which is necessary for the power amplifying section B to generate the maximum output, for example, 3 W is applied. Further, a signal which is added to an output control input terminal 71 is supplied to the transistor 62 through a power control circuit 72 and a collector voltage of the transistor 62 is controlled. The transistor 65 at the final stage is excited by an output voltage of the transistor 62 and an electric power is output from the output terminal 68.
However, according to the conventional transmission output amplifier shown in FIG. 3, since a voltage or current of the transistor is controlled in the power amplifying section B, when the power amplifying section B is excited, it is necessary to excite the power amplifying section B by an electric power such that the maximum output (3 W) of the power amplifying section B can be stably output. However, since such an electric power is larger than the minimum output of the power amplifying section B, the power amplifying section B is operated as an attenuator. According to the conventional control method, although it is rational in case of transmitting the maximum electric power from the power amplifying section B, there is a problem such that a battery cannot be effectively used when a small electric power is transmitted, since electric power consumption doesn't decrease.
On the other hand, a transmission power control circuit having a construction such that a variable amplifier is arranged in front of a power amplifier is disclosed in JP-A-63-77205. According to such a circuit, however, an electric power consumption of a power amplifier cannot be reduced when a small electric power is generated.